It is known to use casting to produce a wide range of components with complex shapes that would be otherwise difficult or uneconomical to manufacture by other methods. Molten material is poured into a mould which defines the shape of the component. The material is then allowed to cool and solidify in the shape of the mould. Where the material has a melting point well above standard ambient temperature and pressure (SATP) (which is typical for most metals), the pouring of the molten material takes place within a furnace. It is known to control the cooling of the molten material in the mould to control the microstructure of the solidified material.
It is known to provide multiple components simultaneously by arranging a plurality of moulds in a single assembly. The moulds are connected by a tree-like network of casting channels through which molten material from a casting cup can be fed to the multiple moulds simultaneously. Once filled, the moulds are collectively drawn from the furnace in a controlled manner.
In, for example, turbine blades it is desirable to provide a single crystal component. This is achieved through a process of “directional solidification” wherein control is exerted over the nucleation and the growth of single crystals in a molten metal as it passes from its liquid state to a solid state. The purpose of such directional solidification is to avoid the negative effects of grain boundaries within the solidified component. One form of directional solidification is single crystal directional solidification ensuring that the part solidifies as a single crystal, so as to minimise the inclusion of grain boundaries, most especially high angle grain boundaries, in the solidified component.
Techniques for producing single crystal components are well known. One example is known as the Bridgman-Stockbarger technique. The mould may contain a seed crystal to initiate a single grain or crystal growth and is gradually withdrawn from the furnace in a direction opposite to that of the desired crystal growth such that the temperature gradient within the molten material is effectively controlled. As an alternative to a seed crystal, a grain selector may be used. The latter typically takes the form of a geometrically designed grain selector cavity at a bottom end of the mould. The shape of the grain selector cavity encourages monocrystalline growth and can be sacrificed in a machining operation subsequent to the casting process.
FIG. 1 shows in schematic a known apparatus for the simultaneous manufacture of multiple cast components using a directional solidification process. As shown in the Figure the apparatus comprises a pouring cup 1 into which molten material M is poured. A plurality of feed channels 2 extend radially around the centrally arranged cup 1 to a top end of the moulds 3. Molten material M poured into the cup 1 flows along the feed channels 2 and into the moulds 3. Each mould 3 is provided with a seed crystal 4 at a bottom end. Beneath the bottom end of the moulds 3 is a chill plate 5 which is maintained generally at a temperature below the melting point of the material M creating a temperature gradient from the bottom to the top of the moulds 3. The moulds are enclosed by a heat source 6 which encircles the cup 1 and mould 3 assembly. With the moulds filled, the assembly is drawn in a controlled manner out of the heat source in the direction of arrow A to ensure directional solidification from the bottom of the moulds 3 (where the seed crystal 4 is arranged) to the top of the moulds 3. The combination of a single crystal seed 4 with the controlled cooling encourages growth of a single crystal structure in the semi-molten casting.
For a component of uniform shape and cross section, consistent directional solidification is relatively simple to achieve, however, many components are non-uniform in shape and cross section and so provide differing radiation heat effects at various points throughout the solidifying component. Due to difficulties in controlling the temperature gradient throughout changes in the shape and cross section an unacceptable occurrence of defects in the solidified component can result.
Moulds for the described apparatus may be formed using the so called “lost wax” or “investment casting” method (though other methods may be used). In this method, a pattern of the desired component shape is formed from a wax or other material of low melting point. The wax pattern is coated in ceramic slurry which is subsequently dried and fired to form a ceramic shell around the wax pattern. The wax can then be heated and removed to provide a mould, the cavity of which defines the desired component shape.
US 2004/0163790A1 proposes the inclusion of “deflector elements” which comprise localised extensions of the mould adjacent to smaller cross-sections of the mould and are arranged substantially orthogonal to the direction of desired solidification. The deflectors may be coated with a heat emissive material and serve to deflect heat back to adjacent smaller cross-sections thereby slowing their rate of cooling to a rate which better matches the cooling rate in larger cross sections of the mould.
US 201510224568A1 proposes a disc-like heat shield which extends in a radial direction from centre fine of the apparatus and around each mould. The heat shield is thus arranged orthogonal to the desired direction of solidification which serves to preserve heat generally in the region of the moulds. For elongated moulds, US 2015/0224568A1 proposes multiple (two) such heat shields axially separated along the length of the moulds.